Well unless your good at climbing poles in the dark and working on live wiring, you might try visiting a local Re-Store and see what they have, they do show up from time to time, put an ISO on Kijiji/Craigslist, phone your city/county, state ask for there surplus sales and give them a call or visit there website/store. Keep an eye on ebay, or make your own from a HPS/MH model, just swap out the gear..

Where are you located? I have a 175w MV cobrahead that I am currently refurbishing and when it is done I might be interested in selling it. It would be around $30 plus whatever shipping costs, which shouldn't be too much since it doesn't actually weigh that much. Otherwise, if you get an HPS cobrahead, you can get a cheap 175w mercury vapor ballast here.

Also, check your local Craigslist frequently. With the amount of LED replacements going on, cobraheads are getting a lot easier to get. They also turn up on eBay quite often and every so often (around this time of the year) they show up for pretty cheap.

Ok the Re-Store by me, I've seen it all, from explosion proof fixtures, highbay, lowbay, floodlights, spotlights, street lighting, crates of ballasts, I visited once and they literately had about a 1000 HID light bulbs, it all kinda depends on the day, about the only things I've never seen there is LPS gear/fixtures but I did score a few LPS tubes, I basically go there every weekend to see if they have anything that's new...

I get my cobra heads dirt cheap, al u have to do is find a metal recycling yard that let u buy scrap metal for reuse projects, i can get them for less than 5 bucks And some times i can save lamps and PC and also street light arms as well. Scrap yard should be happy to sell it to ya as they know they can make little bit more Money off of ya . Best time to go is when scrap yard is not busy. Me best time to go is noon as u have about 1 hour to look around while they go for lunch.